What i understood is you want to have a gauge chart which shows you the percentage usage of the category DBS1 in comparision with over all usage. And that too only for current Month and Year. If yes then try this expression in guage chart.

And go to Number tab of the chart properties and select the show in percentage option.

Bonus Answer. When you are taking the percentage it doesnt matter whether the data is in KB or in MB. For example If I have 10240 KB / 1024000 KB will give me 10% and same if i Use in MB 10 MB / 100 MB will also give me 10%.

Thank you for your answer but i need the gauge to show the amount of data used in the DBS1 category compared by the maximum availablein the DBS1 (max available would be bundle_size_mb * amount of DBS1 connections in database)

or

the amount of data used in MB on average

i am guessing the AVG function is a better solution, but i do not know for sure